It is known in the art that the crystal orientation of ferroelectric materials such as lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3) exhibit an inherent polarization which describes the atomic orientation (or ferroelectric polarization axes) of the material structure. It is also known that the polarization dictates the sign of nonlinear effects of the material and when it is aligned in a certain way, the material is useful for performing functions such as optical frequency doubling of incident light (e.g., converting infrared light having a wavelength of 0.8 .mu.m to blue light having a wavelength of 0.4 .mu.m), or for surface acoustic wave generation.
As used herein, the terms "poling" and "poled" refer to the act of orienting the ferroelectric polarization axes of the material, or the process by which the orientation of these axes is changed. Also, unless otherwise specified, the term "polarization" refers to the aforementioned ferroelectric material characteristic.
In one optical application, i.e., optical frequency doubling, a light wave is launched into one end of a ferroelectric material which has a polarization aligned perpendicular to the travelling wave and regions of the material are poled in alternating directions. A layer of the material along its surface provides a waveguide for the light to travel within (e.g., 10 .mu.m thick). The changes in direction of polarization of the material creates an alternating pattern (also called spatially alternating periodic ferroelectric domains).
Three known fabrication processes for providing a polarization pattern on a material are: titanium indiffusion (Ti in-diffusion), dilithium oxide (Li.sub.2 O) out-diffusion, and electron beam bombardment. The Ti in-diffusion process comprises first applying titanium to the +z surface of the material (i.e., the +z-axis being normal to the surface being poled) and then heating the material by convection (i.e., in an oven) to a temperature of 950.degree. to 1100.degree. C. for LiNbO.sub.3. Heating the material causes the titanium to "in-diffuse" into the surface, thereby permanently reversing the polarization direction in the titanium in-diffused regions.
The Li.sub.2 O out-diffusion process comprises first applying an evaporation protective coating or mask with holes to the +z surface of the LiNbO.sub.3 and then heating the material to 1000.degree. C. in a dry inert atmosphere (e.g., argon (Ar)). Li.sub.2 O evaporates out of the material in the regions uncovered by the coating thereby reversing the polarization in that region.
The electron beam bombardment method comprises first attaching thin, e.g., 500 Angstroms (.ANG.) Nicrome (i.e., NiCr or Nickel Chromium) electrodes (i.e., one that passes electrons freely) on opposite ends of the material, e.g., top and bottom for a z-cut material, between which the polarization is desired, then placing a thin (approximately 4000 .ANG.) mask of gold on top of the top electrode, the mask having holes where polarization reversal is desired. The mask does not pass the electron beam (i.e., it absorbs electrons), thereby blocking the region of the material covered by the mask from exposure to the beam. Next, a voltage is applied across the electrodes such that an electric field of approximately 10 v/cm exists therebetween. While the electric field is applied, an electron beam is applied to the material in a vacuum. This beam creates a "transient vacancy" in the crystalline structure and allows the lithium atoms to move, thereby forcing the crystal to be poled in the direction of the applied electric field.
The aforementioned techniques provide selective poling along the surface of the material which does not extend uniformly from one side of the waveguide, along the z-axis, to the other (i.e., does not provide a "rectangular" grating) and thus they do not provide maximum performance efficiency for functions such as optical frequency doubling. Furthermore, these techniques must all be performed on pre-poled material, i.e., a material that has a predefined direction of polarization all pointing in the same direction. Thus, they provide alternating poled regions by merely reversing the existing polarization in certain areas and leaving other areas alone, thereby creating the desired bidirectional poling pattern having a controlled "period" between changes in direction. Moreover, these techniques are currently performed only on z-cut material, i.e., a material whose polarization axis (z-axis) is normal to the surface to be poled.
If selective poling of localized regions on non z-cut material were available, that obviated the aforementioned shortcomings, it would not only improve the efficiency of optical functions such as optical frequency doubling, but would also open up a whole new realm of possible applications yet to be discovered.
It is known that a bulk sample of non z-cut material may be poled using convection heating and an electric field. This technique comprises, first attaching electrodes to opposite ends of the material between which polarization is desired, then heating the material (e.g., in an oven) to a temperature above the Curie temperature (1150.degree. C. for LiNbO.sub.3 and 610.degree. C. for LiTaO.sub.3) which, as is known, unpolarizes the material (i.e., exhibits random orientation of the polarization axes) and allows the polarization to be set by the presence of an electric field. A voltage is applied across the electrodes thereby creating an electric field therebetween, and, while the electric field is applied, the temperature of the material is reduced below the Curie temperature which fixes the polarization in the direction of the electric field, i.e., the polarization in the material remains after the electric field is removed. Although this process may be used with either pre-poled or unpolarized material, an entire sample of material (i.e., bulk samples such as in boule form) must be poled all in the same direction. Thus, this process cannot be used to provide the aforementioned selective spatially alternating pattern which is desired for many applications.